1. Field of the Invention
The present invention generally relates to a data cache architecture and a cache algorithm and, more particularly, to a data cache architecture for a NAND type flash device and a cache algorithm used in the data cache architecture to minimize the program/erase count of the NAND type flash device.
2. Description of the Prior Art
In a NAND type flash storage application product, the program/erase count is an important factor that determines the lifetime, referred to as mean time between failures (MTBF), of the flash storage application product. Unfortunately, the specification of the program/erase count decreases dramatically due to the technological progress in semiconductor processing (for example, from 65-nm process to 45-nm process).
In the prior art cache algorithm, a cache can be divided into an instruction cache and a data cache, which are used to improve the instruction code or data fetch time. That is, a CPU can access the instruction code or data from the instruction cache or the data cache directly and does not need to fetch from the main storage (external memory) with longer access time.
For a cache algorithm, the cache size, the cache tag memory size and the cache hit rate are some important factors to evaluate a cache algorithm. The larger the cache size, the more hit rate can be achieved. But also, more cache memory cost is needed and more complex cache tag memory control hardware is needed.
In the prior art cache algorithm, the cache memory is uniformly partitioned. For example, for the 32-bit CPU, the instruction cache unit is 32-bit. If the total cache size is 8 KB, then there are 2K (8 kB/32 bit) cache entries. If the total cache size is 16 KB, then there are 4K cache entries, which allows more hit rate.
From a flash memory point of view, the flash memory has an important characteristic that the data in one flash block can not be “overwritten”. If a user wants to overwrite the data, this flash block must be erased first and then it can be programmed (written), which will induce one program/erase count. For an MLC (multiple level cell) flash memory, another characteristic is that a user has to write page data sequentially in one “block”. That is to say, the user has to write page—1, page—2, . . . sequentially in one block, instead of writing page—4 first and then writing page—2 because it is “reverse write”, which will induce data error in the flash memory.
However, the host sometimes reversely writes data in one block. To solve this problem, the user has to “cache” the data first in the cache memory and write back to the flash memory when one block of data is full. By using this method, we can easily solve the “reverse write” and “data overwrite” issues of the flash memory and reduce the program/erase count of the flash memory, which can improve the flash memory lifetime.